The present application claims foreign priority based on German application 197 46 972 filed on Oct. 24, 1997.
1. Field of the Invention
The present invention relates to the area of lithotripters; more particularly, a lithotripter electrode having an automatically adjusting spark gap.
2. Description of Related Art
Lithotripters exist for the contact-free destruction of concrements, e.g. kidney stones, in living bodies. Such devices are also used for the treatment of orthopedic ailments such as heal spurs and tennis elbow as well as non-union of bone problems. Lithotripters and related hardware are described in a number of patents; all of those mentioned below are hereby incorporated by reference.
Lithotripters use an electric underwater spark to generate the shock waves necessary to effect treatment. The spark is generated by an electrode usually mounted in a reflector that is used to focus the shock waves. Examples of these attempts may be found disclosed in U.S. Pat. Nos. 4,608,983 and 4,730,614.
In general, shock wave generation uses a spark produced by a discharge between electrodes. The discharge across the spark gap results from the discharge of an electrical capacitor. Varying the amount of the charging voltage of the capacitor regulates the shock wave energy. A larger or smaller voltage results in the formation of a stronger or weaker spark and thus modifies the strength of the shock wave and the size of the therapeutically active focus and thus in turn the applied shock wave energy.
It is desirable to provide a broad energy spectrum because of the various energy levels of shock waves used to treat different ailments. However, the voltage cannot be varied at will without replacing the electrode assembly because the spark gap, the gap between the electrodes, controls the discharge process. A wider gap requires a larger minimum voltage to bridge the distance between the two electrodes with a spark.
Early lithotripter electrodes used a fixed spark gap. One disadvantage to a fixed-gap electrode is that the electrodes slowly burn away after repeated use, thus increasing the spark gap distance and requiring a greater amount of voltage to generate a spark. But the larger gap and larger minimum voltage produces a stronger shock wave. One invention intended to resolve the electrode burn off issue is disclosed in U.S. Pat. No. 4,809,682.
Another disadvantage is that a low energy shock wave requires a low amount of voltage to be used with a relatively narrow spark gap while a high-energy shock wave requires a large amount of voltage to be used with a relatively wide spark gap. Accordingly, low energy shock waves could not be generated immediately following treatment using high-energy shock waves and vice versa without wholesale replacement of the electrode assembly. If an electrode assembly with a relatively small spark gap is used with a higher voltage, an energy-inefficient spark is produced because a portion of the energy bleeds off into the surroundings and is transformed into acoustic energy while another portion is transformed into heat energy and does not contribute to the formation of the shock wave. In other words, the proper voltage applied to the capacitor must be matched with a proper spark gap to produce an efficient shock wave of the desired energy level.
Another disadvantage with some lithotripter electrode assemblies is the inability to easily exchange one set of electrodes for another. For example, if the electrodes are to be reconditioned or refurbished, electrodes that are permanently attached cannot be removed and replaced.
Subsequent to the disclosure of fixed-gap electrode assemblies, adjustable gap assemblies were invented to overcome the difficulties associated with fixed-gap assemblies. One type, as disclosed by Patent EP 0.349.915 suffers from the disadvantage that it must be adjusted manually; another type, disclosed in U.S. Pat. No. 4,730,614 can only be adjusted in one direction.
Accordingly, there remains a need for an improved, self-adjusting lithotripter electrode assembly that allows a variety of energy levels to be employed, compensates for electrode bum-off, and increases the overall life of the electrode assembly.